Several varieties of this kind of product exist, and all require great care.
The principal problem is that adding a stock to a pistol usually creates an illegal short-barreled rifle: Penal Code 17170
As used in Sections 16530 and 16640, Sections 17720 to 17730, inclusive, Section 17740, Article 1 (commencing with Section 27500) of Chapter 4 of Division 6 of Title 4, and Article 1 (commencing with Section 33210) of Chapter 8 of Division 10 of Title 4, "short-barreled rifle" means any of the following: (a) A rifle having a barrel or barrels of less than 16 inches in length. (b) A rifle with an overall length of less than 26 inches. (c) Any weapon made from a rifle (whether by alteration, modification, or otherwise) if that weapon, as modified, has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length. (d) Any device that may be readily restored to fire a fixed cartridge which, when so restored, is a device defined in subdivisions (a) to (c), inclusive. (e) Any part, or combination of parts, designed and intended to convert a device into a device defined in subdivisions (a) to (c), inclusive, or any combination of parts from which a device defined in subdivisions (a) to (c), inclusive, may be readily assembled if those parts are in the possession or under the control of the same person.
Note that paragraph (e) is an instance where California does have a 'constructive possession' clause - possession of both the kit or device and a pistol it might fit is itself illegal.
Federal law, National Firearms Act, also requires registering Short Barreled Rifles (SBR); BATFE notes
Installation of a barrel greater than 16 inches in length (SBR) or 18 inches in length (SBS) will remove the firearm from the purview of the NFA provided the registrant does not maintain control over the parts necessary to reconfigure the firearm as a SBR or SBS.
That is, you could avoid the NFA problem with a long enough barrel, but you could not keep any barrels for that pistol shorter than the 16 inch + version.
See also the ATF RULING on converting pistol to rifle to pistol.